Semiconductor devices such as semiconductor integrated circuit devices have been mass-produced by repeatedly using an optical lithography process in which a mask serving as a template having a circuit pattern drawn therein is irradiated with exposure light to transfer the circuit pattern onto a semiconductor substrate (hereinafter, referred to as “wafer”) via a reduction optical system.
Recently, semiconductor devices have been more and more miniaturized, and methods of further shortening the exposure wavelength of optical lithography to improve resolution have been studied. More specifically, ArF lithography that uses argon fluoride (ArF) excimer laser light having a wavelength of 193 nm as a light source has been developed so far. However, in recent years, development of lithography that uses EUV light (wavelength=13.5 nm) having a much shorter wavelength has been underway.
In the wavelength region of the above-described EUV light, conventional optical-lithography transmissive masks cannot be used in relation to optical absorption of substances. Therefore, as a mask blank for EUV lithography, for example, a multilayer reflecting substrate utilizing reflection caused by a multi layer film, in which Mo (molybdenum) films and Si (silicon) films are alternately stacked, is used. The reflection caused by the multilayer film is the reflection that utilizes a type of interference.
An EUV lithography mask (hereinafter, referred to as EUV mask or simply mask) is composed of a multilayer-film blank in which stacked films of, for example, Mo films and Si films are deposited on a quartz substrate or low-thermal-expansion glass substrate (LTEM: Low Thermal Expansion Material) and an absorber pattern formed on the multilayer-film blank.
In EUV lithography, since the mask is a reflecting type and the exposure wavelength is as extremely short as 13.5 nm, even an extremely slight abnormality in height corresponding to a fraction of the exposure wavelength causes a local difference in the reflection rate, and a phase defect occurs in the pattern transferred onto a wafer.
Most of above-described phase defects are caused by pits created in substrate polishing and particles on a substrate which cannot be removed even by cleaning. These phase defects belong to opaque defects, and it is difficult to recover them because they are defects of a reflecting member.
In view of such facts, a method of depositing a polysilicon film or a multilayer film for planarization on a polished quartz substrate or low-thermal-expansion glass substrate and then forming a multilayer film serving as an original reflecting film on the film to fabricate a mask blank or mask has been known.
In the case of the method in which an original multilayer film is to be formed on a multilayer film for planarization, in order to vent the influence of the underlying multilayer film from appearing as changes in the reflection-rate level of EUV light or the local reflection rate, a method of applying thermal treatment to the underlying multilayer film to cause mixing at the interface of the two multilayer films (for example, see Japanese Unexamined Patent Application Publication No. 2007-109971 (Patent Document 1)), a method of providing an intermediate film composed of an absorber which suppresses reflection of EUV light between the two multilayer films for example, see Japanese Unexamined Patent Application Publication No. 2007-109968 (Patent Document 2)) and others have been proposed.
As typical methods of a mask-blank defect inspection carried out in a stage before a step of forming the absorber pattern on the multilayer film, a laser inspection method of obliquely irradiating a mask blank with laser light and detecting foreign matters from diffusely reflected light thereof and an exposure-wavelength (at wavelength or Actinic) defect inspection method of detecting defects by using EUV light having the same wavelength as the wavelength of exposure light have been known.
Furthermore, examples of the above-described exposure-wavelength defect inspection method include a method that uses a dark-field image (for example, see Japanese Unexamined Patent Application Publication No. 2003-114200 (Patent Document 3)), X-ray microscopy that uses a bright field (for example, see Japanese Unexamined Patent Application Publication No. 6-349715 (Patent Document 4)), and a dark-field bright-field combination method that detects defects by using a dark field and carries out defect identification with a bright-field system using a Fresnel zone plate (for example, see US Patent Application Publication No. 2004/0057107 (Patent Document 5)).